Campanelle with Walnuts, Ricotta, and Lemon

INGREDIENTS

• 1/2 cup plus 2 tablespoons extra-virgin olive oil
• 4 large garlic cloves
• Coarse salt and freshly ground pepper
• 1 pound campanelle pasta
• 1 cup packed fresh flat-leaf parsley leaves, divided, plus 2 tablespoons finely chopped parsley stems (from 1 small bunch)
• 2 heaping tablespoons finely grated lemon zest (from 3 large lemons)
• 4 ounces walnuts, toasted well and roughly chopped (1 1/4 cups)
• 1 cup fresh ricotta cheese